The Bacteroides are one of the numerically dominant genera of the human intestinal microbiota where they serve as mutualistic symbionts providing beneficial functions to the mammalian host. If Bacteroides fragilis gains access to the otherwise sterile peritoneal cavity due to a breach in the intestinal barrier from natural, traumatic, or surgical circumstances, it can become an opportunistic pathogen. In fact, B. fragilis is one of the most commonly isolated anaerobes from clinical infections. The capsular polysaccharides of B. fragilis have been shown to be instrumental in the ability of this organism to both provide symbiotic benefits to the mammalian host in its natural intestinal niche, and to cause disease if it gains access to extraintestinal sites. The long term objective is to understand how this complex set of capsular polysaccharides is synthesized in both the intestine and peritoneal cavity to better appreciate their contributions to symbiosis and virulence. The aims will address the many levels of regulation of the synthesis of the capsular polysaccharides. In particular, Aim 1 will analyze the mechanisms governing the ability of a set of factors to repress transcriptional termination of their respective polysaccharide biosynthesis loci. Aim 2 will analyze the mechanisms by which a set of novel transcriptional repressors are able to interfere with the expression of specific capsular polysaccharides. Aim 3 will address the synthesis of a previously undescribed extracellular polysaccharide which will be analyzed at the phenotypic and transcriptional levels. Lastly, the expression of each of the polysaccharides, dictated by promoter orientation due to DNA inversions, will be studied from bacteria isolated from relevant in vivo sites.